1. Field of the Invention
The present invention relates to a plasma CVD apparatus for uniformly depositing a semiconductor thin film on a substrate, with excellent excellent electric characteristics and to a method for manufacturing a thin film. More particularly, the invention relates to a method and apparatus for continuously fabricating a photovoltaic element comprising a microcrystal thin film on an elongated substrate.
2. Related Background Art
As a method for continuously forming a semiconductor functional deposited film, used for photovoltaic elements or the like, on a substrate, the specification of U.S. Pat. No. 4,400,409 discloses the plasma CVD process employing the roll-to-roll method.
This process is described as being capable of continuously forming an element having a semiconductor junction by using an elongated beltlike member as a substrate, and continuously conveying the substrate in the longitudinal direction thereof while depositing necessary conductivity-type semiconductor layers hereon in a plurality of glow discharge regions.
A schematic diagram of a conventional deposition apparatus is shown in FIG. 2. A beltlike member 2000 is conveyed by magnet roller 2005. When electric power is applied between cathode electrode 2002 and anode electrode 2004, material gas 2003 is decomposed to generate a plasma in a glow discharge space 2006 and to form a film on the beltlike member 2000. In FIG. 2 the arrow 2003 represents the flow of the material gas.
The above apparatus, however, has a problem that it is not easy to maintain uniform discharge states with good reproducibility during a period of several hours for depositing the film over the entire length of the elongated substrate.
Further, a light-incident-side doped layer of amorphous silicon solar cell is desirably microcrystallized due to demand for improvement in optical transparency.
As conventional techniques for forming a microcrystalline p-type semiconductor layer or n-type semiconductor layer, there are a method for mixing phosphine (PH3), diborane (B2H6), or the like as a dopant in silane (SiH4) or the like being the material gas and for further diluting it with a large amount of hydrogen (H2) (in the dilution rate of 10 to 100 or more), and a method for applying high radio-frequency (RF) power, but they were not enough to stably form the microcrystalline film. The reason is that excitation and decomposition of the material gas is promoted only in a certain localized portion in the proximity of the cathode electrode. Also, these methods consume a lot of both raw-material gas and power, thus being disadvantageous from a viewpoint of cost.
Further, there is another conventional technique for positively applying a positive potential (bias) to the cathode electrode using direct-current (DC) power supply or the like. However, since such a system employs the secondary means of DC power supply, it is a system to permit direct current to flow into plasma discharge. Therefore, abnormal discharge such as sparks will occur with increasing DC voltage bias. It was thus very difficult to maintain stable discharge as suppressing the abnormal discharge. Accordingly, it was doubtful whether application of the DC voltage to the plasma discharge was effective. This is because the system is one in which the DC voltage is not separated from the direct current. In other words, it has been desired to have a means for effectively applying only the DC voltage to the plasma discharge.
An object of the present invention is to provide a method and deposition apparatus capable of forming a semiconductor thin film which is spatially uniform and and excellent in electric characteristics, with good reproducibility, and at a high deposition rate.
The deposition apparatus of the present invention is arranged so that a surface area of an RF power applying cathode electrode disposed in a glow discharge space, in a space in contact with discharge is greater than a surface area of the whole of a ground electrode (anode electrode) including a beltlike member in the discharge space.
This solves the problem that excitation and decomposition of material gas is promoted only in a certain limited portion near the cathode electrode, which was the defect in the conventional technology. Namely, the above-stated excitation and decomposition of material gas is promoted in the entire discharge space, more specifically, on the anode electrode side including the beltlike member. As a result, a high-quality thin film can be deposited efficiently and at a relatively high deposition rate on the beltlike member.
By this structure, the potential (self-bias) of the cathode electrode disposed in the glow discharge space can be maintained automatically at a positive potential with respect to the ground (anode) electrode including the beltlike member.
As a result, the bias is applied in the direction of irradiation of the positively charged ions to the deposited film on the beltlike member, and therefore, the ions existing in the plasma discharge are accelerated more efficiently toward the beltlike member. Thus, they effectively give energy to the surface of the deposited film by so-called ion bombardment. Therefore, since structural relaxation of the film is promoted even at relatively high deposition rates, the microcrystalline semiconductor film can be formed at the relatively high deposition rates with good efficiency, with high uniformity, and with good reproducibility.
The method and apparatus of the present invention can be applied not only to film formation of the light-incident-side doped layer, but also to film formation of the i-layer and opposite-side doped layer with the i-layer inbetween. These films do not always have to be microcrystallized, but application of the apparatus and method of the present invention promotes the structural relaxation more, thus permitting formation of films with fewer defects.
The potential (self-bias) of the cathode electrode upon glow discharge is desirably +5 V or more upon deposition of the i-layer and +30 V or more upon formation of the doped layers. More desirably, it is maintained at +100 V or more.
Further, the apparatus may be so arranged that it has means capable of introducing different gas species independently of each other through a plurality of gas inlet tubes to one film-forming container, at least one of which is a dedicated inlet tube for supplying a dopant or an additive such as germanium, carbon, nitrogen, or oxygen and has structure capable of supplying the dopant or additive to near the surface of the beltlike member and another of which is for supplying a material gas such as silane becoming a source for forming the film and a diluent gas such as hydrogen and has structure capable of supplying the gases to a region relatively apart from the beltlike member.
When the dopant is supplied to near the surface of deposited film, more doping gas molecules adhere to the surface of the deposited film and the ions effectively give energy to the adhering doping gas molecules, which improves the doping efficiency of dopant and which, at the same time, enhances the quality and fineness of the film. Therefore, a low-resistance microcrystalline semiconductor thin film can be obtained relatively easily.
When the additive is supplied near the surface of the deposited film, more additive gas molecules adhere to the surface of the deposited film and the ions effectively give energy to the adhering additive gas molecules, which results in decomposing and activating the additive more effectively, so as to promote the structural relaxation of atoms, thereby enhancing the quality and fineness of film.
Examples of the raw-material gas suitable for deposition of the p-layer, n-layer, and i-layer of photovoltaic element of the present invention are gasifiable compounds containing silicon atoms, gasifiable compounds containing germanium atoms, gasifiable compounds containing carbon atoms, etc., and mixtures of gases of the mentioned compounds.
Specific examples of the gasifiable compounds containing silicon atoms are SiH4, Si2H6, SiF4, SiFH3, SiF2H2, SiF3H, Si3H8, SiD4, SiHD3, SiH2D2, SiH3D, SiFD3, SiF2D2, SiD3H, Si2D3H3, and so on.
Specific examples of the gasifiable compounds containing germanium atoms are GeH4, GeD4, GeF4, GeFH3, GeF2H2, GeF3H, GeHD3, GeH2D2, GeH3D, Ge2H6, Ge2D6, and so on.
Specific examples of the gasifiable compounds containing carbon atoms are CH4, CD4, CnH2n+2 (n is an integer), CnH2n (n is an integer), C2H2, C6H6, CO2, CO, and so on.
Examples of gas containing nitrogen atoms are N2, NH3, ND3, NO, NO2, and N2O.
Examples of gas containing oxygen atoms are O2, CO, CO2, NO, NO2, N2O, CH3CH2OH, CH3OH, and so on.
Examples of the substance introduced into the p-type layer or the n-type layer in order to control the valence electrons in the present invention are the atoms in Group III and the atoms in Group V in the periodic table.
Materials effectively used as a starting substance for introduction of the atoms in Group III in the present invention, specifically for introduction of boron atoms, are boron hydrides such as B2H6, B4H10, B5H9, B5H11, B6H10, B6H12, and B6H14, boron halides such as BF3 and BCl3, and so on. Additional examples are AlCl3, GaCl3, InCl3, TlCl3, and so on. Particularly, B2H6 and BF3 are suitable.
Materials effectively used as a starting substance for introduction of the atoms in Group V in the present invention, specifically for introduction of phosphorus atoms, are phosphorus hydrides such as PH3 and P2H4, phosphorus halides such as PH4I, PF3, PF5, PCl3, PCl3, PBr3, PBr5, and PI3, and so on. In addition, other examples are AsH3, AsF3, AsCl3, AsBr3, AsF5, SbH3, SbF3, SbF5, SbCl3, SbCl5, BiH3, BiCl3, BiBr3, and so on. Particularly, PH3 and PF3 are suitable.
Further, the foregoing gasifiable compounds may be introduced into a deposition chamber as being diluted properly with a gas such as H2, He, Ne, Ar, Xe, or Kr.